1. Field of the Invention
This invention relates to a stepping motor and, more particularly to a novel improvement therein in which a stator magnet is provided in the stator to increase the torque significantly without increasing the size of the rotor for the stator.
2. Prior Art
A variety of the above type of stepping motor have been employed in the art. FIGS. 1 to 4 show the construction of the stepping motor now being prepared in the company of the present Assignee and in which a stator disclosed in Japanese Patent Application KOKAI No. 254058/1986 is employed.
In these figures, a stator 1 having stator coils 1A in its yokes 4 is provided on its forward and rear sides with a front cover 2 and a rear cover 3 such that the stator 1 is sandwiched between the front cover 2 and the rear cover 3.
The stator 1 is formed by a large number of laminated press-worked thin silicon steel plates. As shown in FIGS. 2 and 3, it is provided with a number of radially projecting stator yokes 4, the distal ends of which are formed with a number of small teeth 4a.
A rotor shaft 8 carrying a rotor 7 is carried for rotation by a bearing 5 provided in the front cover 2 and a bearing 6 provided in the rear cover 3. A sleeve 9 is interposed between the bearings 5, 6 and the rotor 7.
The rotor 7 is formed by a pair of rotor yokes 10, 10 each having a number of small teeth 10a and rotor magnets 11 provided between the rotor yokes 10.
In operation, when a drive signal is supplied to the stator coil 1A, the rotor performs a step-by-step rotation, under the magnetic interaction between the stator yokes 4 and the rotor 7, with a predetermined angle of step-by-step rotation corresponding to the number of stator poles.
In the above described prior-art stepping motors, inasmuch as the stator is formed by a large number of laminated silicon steel plates, the magnetic fluxes emanating from the rotor are distributed among the small teeth of the stator, such that the stator yokes associated with the rotor magnets are supplied with magnetic fluxes emanating from the portions facing the small teeth of the rotor. It is therefore extremely difficult or even impossible with the above described prior-art stepping motor to develop a larger effective motor torque without altering the shape of the stator or the rotor.